When a vehicle with automatic transmission has stopped, it must be held by the driver using the brake, since when drive is engaged, because of the converter, the vehicle has a tendency to move forward slowly (creep). The load on the driver can be relieved in that once the necessary braking effect, generally the brake pressure, has been applied, it is held substantially constant. This can be achieved for example in that the brake pressure initiated by the driver is blocked in the wheel by a valve between the brake master cylinder and the wheel brake cylinder as soon as the vehicle has stopped.
For this a vehicle speed sensor detects the vehicle stoppage. The driver can then remove his foot from the brake while the wheels remain braked. The brake pressure in the wheels is dissipated as soon as the driver activates the gas pedal and hence signals his wish to move away. Such a system for creep suppression is known for example from DE-OS 43 32459.
Starting a vehicle with manual transmission on a hill is a complex process which requires the combined use of drive and clutch pedals in cooperation with operation of the handbrake. The difficulty lies in metering the braking effect or brake moment during the start-up process so that the vehicle does not roll in the wrong direction until the drive moment transmitted via the manual transmission is sufficient for the actual start. There are many proposals for relieving the load on the driver in this situation. In vehicles with hydraulic brake systems for example the wheel brake pressure can be separated from the master brake cylinder pressure via a control valve. The brake pressure once applied by the driver is thus maintained at the wheels even if the driver no longer operates the brake pedal. This process is activated with a special switch. The driver can now initiate the starting process without having to worry about the brake. The control valve is opened as soon as a vehicle movement is detected via a change in the rotary position of the drive shaft. Reference is made to DE-OS 38 32 025 for such starting aids (“hillholder”).
A generic pressure-medium-activated brake device with creep suppression and hill start assistant (hillholder) function is discussed for example in DE 196 25 919 A1. There, to sense the activation conditions for the operating mode “creep suppression” or “hillholder”, only those signals are used which are present in any case in vehicles with an ABS and/or TCS system. The creep suppression or starting aid functions can then be integrated in an ABS/TCS system with no additional sensors. The control routines for the creep suppression and hill start (hillholder) functions are each implemented, according to FIGS. 2a and 2b of this publication, in a separate control unit (reference numerals 204a and 204b). Furthermore as well as the brake pressure build-up valves and brake pressure reduction valves (reference numerals 19 or 20), which during wheel slip regulation mode are controlled by a control unit (not shown explicitly there), switchover valves USV1 and USV2 are present which are provided merely to implement the creep suppression and hill start function (hillholder).